In recent years, multi user multi input multi output (MIMO) technology that simultaneously transmits data from a base station provided with a plurality of transmission antennas to a plurality of user terminals has been drawing attention. In multi user MIMO (hereinafter, simply referred to as “MU-MIMO”), countermeasure, such as Zero Forcing (ZF), block diagonalization, or the like, that multiplies a transmission weight by a transmission signal in order to prevent interference between data addressed to a plurality of user terminals, is sometimes used. The transmission weight is a weight that adjusts the phase and the amplitude of the transmission signal and, by deciding the transmission weight in accordance with the channels between base stations and user terminals, interference can be reduced by making the transmission signals addressed to the plurality of the user terminals orthogonal.
In contrast, coordinated multi-point transmission and reception (CoMP) in which a plurality of cells sends and receives signals to and from a single user terminal in a cooperation manner is also actively studied. Furthermore, in order to increase the capacity of the radio communication system, it is conceivable that a plurality of transmitter stations that performs CoMP simultaneously transmits signals to a plurality of user terminals by using MU-MIMO.
In this radio communication system, because the propagation distance between each of the transmitter stations and the user terminals is different, a propagation delay difference is generated. For example, if, at the same time when a signal is transmitted from a certain transmitter station to a user terminal, a signal is transmitted from another transmitter station to the same user terminal, the signal transmitted from the transmitter station whose distance from the user terminal is greater is received later. Thus, if the reception phase of the signal transmitted from a first transmitter station is used as the reference, the reception phase of the signal transmitted from a second transmitter station rotates in a frequency domain. Namely, the signals transmitted from the two transmitter stations are received by the phase differences that are different for each frequency and, depending on the frequencies, the reception signals received from the two transmitter stations interfere with each other. Thus, studies have been conducted on a method of deciding an optimum transmission weight or the like based on the assumption that interference is additionally generated due to an influence of a propagation delay difference.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-39400
Patent Document 2: International Publication Pamphlet No. WO 2013/108906
Patent Document 3: Japanese National Publication of International Patent Application No. 2014-514841
Patent Document 4: International Publication Pamphlet No. WO 2012/108281
Non-Patent Document 1: S. B. Gee, Z. Lei, and Y. H. Chew, “Cooperative Multiuser MIMO Precoding Design for Asynchronous Interference Mitigation”, 2011 IEEE GLOBECOM Workshops (GC wkshps), Dec. 5-9, 2011
Non-Patent Document 2: Takashi Seyama, et al. “A Basic Study on Joint Transmission MU-MIMO in 5G Ultra High-Density Distributed Antenna System” Proceedings of the 2015 the Institute of Electronics, Information and Communication Engineers (IEICE) Society Conference, communication (1), 326, Aug. 25, 2015
As described above, if there is a difference between the propagation delay time between the transmitter stations and the user terminals, the phase difference of the reception signal from each of the transmitter stations differs for each frequency in the user terminals. Thus, the optimum value of the transmission weight that adjusts the phase and the amplitude of the transmission signal is different for each frequency. Thus, if a propagation delay difference is present, it is difficult to sufficiently reduce interference even if the transmission weight is decided by taking into consideration the influence of the propagation delay difference. Namely, even if the transmission weight is decided by taking into consideration the influence of the propagation delay difference, if this transmission weight is uniformly multiplied by the transmission signal in the whole band, the interference of only a part of frequency component is merely reduced.
Thus, because the optimum transmission weight differs in accordance with the frequency, it is conceivable that the band of the transmission signal is divided and an optimum transmission weight is calculated for each of the frequency sections. However, in order to sufficiently reduce the interference, because the transmission signal is divided into a large number of narrow band frequency sections and the transmission weight is calculated for each frequency section, there is a problem in that an amount of process is increased. In other words, by calculating a transmission weight with a high frequency resolution, the amount of process is increased in proportion to the frequency resolution.